1. Field of the Invention
This invention relates to the field of microprocessors and, more particularly, to floating point units within microprocessors.
2. Description of the Related Art
Superscalar microprocessors achieve high performance by executing multiple instructions per clock cycle and by choosing the shortest possible clock cycle consistent with the design. As used herein, the term "clock cycle" refers to an interval of time accorded to various stages of an instruction processing pipeline within the microprocessor. Storage devices (e.g. registers and arrays) capture their values according to the clock cycle. For example, a storage device may capture a value according to a rising or falling edge of a clock signal defining the clock cycle. The storage device then stores the value until the subsequent rising or falling edge of the clock signal, respectively. The term "instruction processing pipeline" is used herein to refer to the logic circuits employed to process instructions in a pipelined fashion. Generally speaking, a pipeline comprises a number of stages at which portions of a particular task are performed. Different stages may simultaneously operate upon different items, thereby increasing overall throughput. Although the instruction processing pipeline may be divided into any number of stages at which portions of instruction processing are performed, instruction processing generally comprises fetching the instruction, decoding the instruction, executing the instruction, and storing the execution results in the destination identified by the instruction.
Microprocessors are configured to operate upon various data types in response to various instructions. For example, certain instructions are defined to operate upon an integer data type. The bits representing an integer form the digits of the integer number. The decimal point is assumed to be to the right of the digits (i.e. integers are whole numbers). Another data type often employed in microprocessors is the floating point data type. Floating point numbers are represented by a significand and an exponent. The base for the floating point number is raised to the power of the exponent and multiplied by the significand to arrive at the number represented. While any base may be used, base 2 is common in many microprocessors. The significand comprises a number of bits used to represent the most significant digits of the number. Typically, the significand comprises one bit to the right of the decimal, and the remaining bits to the left of the decimal. The bit to the right of the decimal is not explicitly stored, instead it is implied in the format of the number. Generally, the exponent and the significand of the floating point number are stored. Additional information regarding the floating point numbers and operations performed thereon may be obtained in the Institute of Electrical and Electronic Engineers (IEEE) standard 754. Floating point instructions, therefore, are instructions having at least one operand of the floating point data type.
Floating point numbers can represent numbers within a much larger range than can integer numbers. For example, a 32 bit signed integer can represent the integers between 2.sup.31 -1 and -2.sup.31, when two's complement format is used. A single precision floating point number as defined by IEEE 754 comprises 32 bits (a one bit sign, an 8 bit biased exponent, and a 24 bit significand) and has a range from 2.sup.-126 to 2.sup.127 in both positive and negative numbers. A double precision (64 bit) floating point value has a range from 2.sup.-1022 and 2.sup.1023 in both positive and negative numbers. Finally, an extended precision (80 bit) floating point number has a range from 2.sup.-16382 to 2.sup.16383 in both positive and negative numbers.
The expanded range available using the floating point data type is advantageous for many types of calculations in which large variations in the magnitude of numbers can be expected, as well as in computationally intensive tasks in which intermediate results may vary widely in magnitude from the input values and output values. Still further, greater precision may be available in floating point data types than is available in integer data types.
Many microprocessor architectures define the floating point instructions and operation thereof in such a way that a coprocessor-style design is favored. For example, the x86 microprocessor architecture defines the floating point in this manner. A coprocessor typically executes instructions provided by the processor relatively independent of the processor. An interface between the floating point unit and the integer portion of the microprocessor is therefore needed which preserves the independence of the floating point unit even though the floating point unit is integrated into the microprocessor.